I don’t know if I’ve said this before, but I find it really amusing that Isis just calls Kaiba ‘Seto’ without an honorific or anything. Like, it’s rude and over-familiar, and she must know that it’s rude and over-familiar because, despite being a foreigner, her Japanese is otherwise neat and polite. It makes me thing somebody more savvy than Seto could’ve called her out on it, except Isis somehow sensed that Seto is the least savvy person on the planet and he was incapable of handling that many variables and politely telling her to stuff it, so she knew she could get away with it. And it amuses me.
Either that or they’re speaking in English. I’m pretty sure Isis knows Japanese at least competently if she’s speaking to Yuugi and the others later, but I tend to think, in a post-canon setting
at the very least, that she and Seto default to speaking English with one another.